1. Field of the Invention
The present invention relates to a cell pack of a direct methanol fuel cell, and more particularly, to a cell pack of a direct methanol fuel cell in which a circuit connecting cells is simplified and which can effectively exhaust byproducts.
2. Description of the Related Art
A direct methanol fuel cell (DMFC), which is a source of future clean energy that can replace fossil energy, has high power density and high energy conversion efficiency. Also, the DMFC can be operated at room temperature and can be made light-weight and miniaturized. Thus, the DMFC has very wide applications including zero-emission vehicles, home generator systems, mobile communications equipment, medical appliances, military equipment and space industry equipment.
DMFCs produce electricity by electrochemical reaction between methanol and oxygen. A unit battery of such DMFCs, that is, a cell, as shown in FIG. 1, is constructed such that a proton exchange membrane 1 is interposed between an anode 2 and a cathode 3. The proton exchange membrane 1 has a thickness of 50 to 200 xcexcm and is made of solid polymer electrolyte. Both of the anode 2 and cathode 3 of such a cell include a support layer for supply and diffusion of fuel and a catalyst layer at which oxidation/reduction of fuel occur.
Carbon paper or carbon cloth is used as the support layers of the anode 2 and the cathode 3 and the support layers are waterproof for supply of methanol as liquid fuel and easy exhaustion of water that is a reaction product.
In the anode 2, methanol, ethanol or isopropyl alcohol and water are reacted to produce protons, electrons and carbon dioxide (oxidation). The produced protons are transferred to the cathode 3 through the proton exchange membrane 1. In the cathode 3, the protons and oxygen are reacted to produce water (reduction).
The following reaction equations 1 and 2 represent reactions occurring in the anode and cathode and the reaction equation 3 represents an overall reaction occurring in the single cell.
CH3OH+H2Oxe2x86x92CO2+6H++6exe2x88x92xe2x80x83xe2x80x83[Reaction equation 1]
{fraction (3/2)}O2+6H++6exe2x88x92xe2x86x923H2Oxe2x80x83xe2x80x83[Reaction equation 2]
CH3OH+3/2O2xe2x86x92H2O+CO2xe2x80x83xe2x80x83[Reaction equation 3]
A theoretical voltage generated in a single cell is approximately 1.2 V. Thus, in order to generate a high voltage, several single cells are stacked and electrically connected in series. Here, as many flow fields and bipolar plates as stacked cells are required for supplying each single cell with fuel and air and to collect generated electricity. Although metal mesh may be typically used as the flow fields, flow fields may be grooved on a graphite block having electrical conductivity, capable of hermetically sealing gas and having a predetermined thickness.
However, in such a case, in order to supply fuel and oxygen continuously throughout stacked cells from the outermost single cell to the innermost single cell without being mixed, the design of a complicated flow field is necessary. For this reason, liquid or gas supplied to the cells is prone to leakage. Also, since many graphite blocks are stacked, hermetic sealing and reduction in size and weight of the stack are difficult to realize, affecting power density. Also, since internal resistance, temperature and humidity of the outermost and innermost parts of the stack are not uniform, single cells are partially subjected to high load, resulting in shortened life of the stack. In spite of such drawbacks, the conventional stack is advantageously adopted for small power density. However, for attainment of low power density, a monopolar cell pack structure overcoming such drawbacks is advantageously adopted.
A conventional monopolar cell pack is constructed such that anodes 2a are disposed at one side of an ion exchange membrane 1a and cathodes 3a corresponding to the anodes 2a are disposed at the opposite side, as shown in FIGS. 2A and 2B. In order to electrically series-connect the respective cells, a connection wire 4 connecting the anode 2a and cathode 3a of neighboring cells must pass through the ion exchange membrane 1a between the anode 2a and the cathode 3a. In this case, a path or hole for passage of the connection wire 4 must be provided in the ion exchange membrane 1a. However, since the path or hole is likely to cause leakage of fuel, a path or hole portion should be sealed. If the connection wire 4 does not pass through the ion exchange membrane 1a, the connection wire 4 must be re-routed outside the cell pack.
As described above, if a connection wire is re-routed outside a cell pack, the length of the connection wire necessarily becomes longer causing a current loss due to an increase in line resistance, resulting in leakage of fuel. Thus, it is necessary to seal a connection wire portion. In the conventional cell pack, since the contact between the current collector and anode or cathode electrode is bad and a contact area is not wide, a current loss is generated due to contact resistance. Another drawback encountered with the conventional cell pack is in that a supply of fuel is hindered by CO2 gas because there is no exhaust path for byproducts, that is, the CO2 gas, resulting in deteriorated activity of electrodes.
FIG. 3 is a schematic diagram of a conventional cell pack 10 disclosed in U.S. Pat. No. 5,925,477.
Referring to FIG. 3, in a state in which some parts of single cells are disposed in a row so as to overlap with neighboring cells, cathodes 13 and 13a of the respective cells are electrically connected in series to an anode 12a of a cell next thereto by current collectors 14 and 14a. According to this structure, flow fields for supplying fuel must be formed on a graphite plate and a fuel path from the outside of cells must be separately provided for a fuel flow among electrodes. Also, since electrodes where electrochemical reactions occur, that is, anodes and cathodes, should be bent, the service life of electrodes is shortened and the manufacturing process thereof is complex.
To solve the above-described problems, it is a first object of the present invention to provide a monopolar cell pack for a direct methanol fuel cell having a simplified electrical connection structure among cells.
It is a second object of the present invention to provide a monopolar cell pack for a direct methanol fuel cell in which fuel leakage of a cell can be effectively suppressed.
It is a third object of the present invention to provide a monopolar cell pack for a direct methanol fuel cell in which gas generated in a cell can be effectively exhausted.
To achieve the first object of the present invention, there is provided a monopolar cell pack for a direct methanol fuel cell including an upper plate and a lower plate spaced a predetermined distance apart from each other, an ion exchange membrane provided between the upper plate and the lower plate, having a first surface and a second surface corresponding to the first surface and having a plurality of single cell regions on the first and second surfaces, a plurality of first anodes installed in each single cell region on the first surface of the ion exchange membrane and a plurality of first cathodes disposed in each single cell region adjacent to each of the anodes, a plurality of second cathodes installed in each single cell region on the second surface of the ion exchange membrane corresponding to the first anodes, and a plurality of second anodes corresponding to the first cathodes, first and second anode current collectors installed on the first and second anodes and each having a fuel passage region, first and second cathode current collectors installed on the first and second cathodes and each having an air passage region, a plurality of first conductive portions electrically connecting the first anode and cathode adjacent to each other on the first surface of the ion exchange membrane, and a plurality of second conductive portions electrically connecting the second anode and cathode adjacent to each other on the second surface of the ion exchange membrane to electrically connect in series cells provided in the single cell regions.
Preferably, fuel supply regions for supplying fuel to the first and second anodes, and air supply regions for supplying air to the first and second cathodes, are provided on the upper and lower plates.
Also, the first and second anode current collectors corresponding to the first and second anodes and cathodes may have a size corresponding to that of each of the fuel supply regions provided on the upper and lower plates, and the first and second cathode current collectors may have a size corresponding to that of each of the air supply regions provided on the upper and lower plates.
The first conductive portion is preferably integrally formed with each of the first anodes disposed on the first surface of the ion exchange membrane and the first cathode electrically connected thereto. The second conductive portion is preferably integrally formed with each of the second anode disposed on the second surface of the ion exchange membrane and the second cathode electrically connected thereto.
Preferably, current collector insertion grooves into which first current collectors are inserted are formed on the inner surface of the upper plate. Current collector insertion groove and conductive portion insertion groove into which the second anode current collectors, the second cathode current collectors, and the second conductive portion connecting these current collectors are inserted, are formed on the inner surface of the lower plate.
A plurality of first gas exhaust channels for exhausting byproducts generated at the first and second anodes are preferably formed on the inner surfaces of the first and second anode current collectors, and a plurality of second gas exhaust channels connected to the first gas exhaust channels are preferably formed on the inner surfaces of the upper and lower plates.